In its more general aspect, the present invention relates to a tire for vehicles.
More particularly, the invention relates to a very high performance tire for vehicles having a curvature ratio not greater than 0.1, the tire being preferably, however not exclusively, used in motor-vehicles capable of providing a high torque and reaching high speeds both in straight and curve.
The known tires of this type comprise:
a carcass structure including a central crown portion and two sidewalls ending in a couple of beads for anchoring to a rim of a wheel;
a belt structure, coaxially associated to the carcass structure;
a tread, coaxially extending around the belt structure, on the tread being defined:
i) an equatorial zone extending on either side of the equatorial plane of the tire, and
ii) two shoulder zones in axially opposite positions with respect to said equatorial zone;
said tread comprising a plurality of transversal grooves including a first shoulder portion and a second equatorial portion.
In the following description and in the appended claims, the terms:
xe2x80x9cgroovexe2x80x9d and xe2x80x9cslitxe2x80x9d are intended to indicate grooves formed on the tire tread having a width greater than and, respectively, equal to or lower than 2 mm;
xe2x80x9clongitudinalxe2x80x9d and xe2x80x9clongitudinallyxe2x80x9d are intended to indicate entities measured along the circumferential development of the tire;
xe2x80x9caxialxe2x80x9d and xe2x80x9caxiallyxe2x80x9d are intended to indicate entities measured along the peripheral surface of the tire in a direction perpendicular with respect to the equatorial plane of the same.
Finally, in the following description and in the appended claims, the various geometrical entities will be measured with reference to the median axis of the elements present on the tread.
As is known, in the production of tires and in particular of tires of the so called very high performance type to be mounted on high-powered sports-cars, the need has always been felt of ensuring an adequate performance of the tire, in spite of the extreme stresses it has to withstand in use, particularly when the same is a sports use.
The Applicant believes that the difficulty of fully satisfying the above requirement is essentially related to the difficulty of limiting the mobility of the blocks (meaning by this term the tread portions delimited between consecutive grooves, both along the axial and along the circumferential direction) present on the tread as the blocks warm up during use.
Up to now the attempts made in the art for ensuring the desired high performance levels of the tire and based on the design of treads provided with variously inclined grooves and variously shaped blocks have not allowed to achieve entirely satisfactory results.
In fact, during the tire rolling, the blocks present on the tread are submitted to a full range of thermal-mechanical stresses, mainly due to the warming of the rubber composition caused by friction and to the compression and shear stresses which tend to bend and deform the blocks, modifying their geometry and causing a degradation of tire performances, in particular during the so-called xe2x80x9cboundaryxe2x80x9d driving.
Besides, in the tires of the known art, a performance degradation of the same after a certain wear of the tread takes place almost always, since the thermal-mechanical stresses imparted to the blocks increasingly modify the geometry thereof which, in its turn, causes an increasingly marked deviation from the desired road behavior.
Tires having no longitudinal grooves are known in the field of motor-cycle tires, for instance as described in copending European patent applications Nos. 97 202 112.5 and 97 202 113.3, i.e. tires showing a high transverse curvature with respect to the corresponding motor-vehicle tires.
As is known, in a tire the transverse curvature is defined by the value of the ratio between the distance of the tread crown from the line passing through the tread end points as measured on the equatorial plane, also known as xe2x80x9ccamberxe2x80x9d of the tread, and the distance between said end points of the tread. In motor-cycles tires, such ratio, indicated in the following as xe2x80x9ccurvature ratioxe2x80x9d usually has a value not lower than 0.15. The curvature ratio of a conventional tire for motor-vehicles has a value equal to about 0.05 and in any case it is never higher than 0.1.
However, in a motor-cycle tire, the limited width of the tread and the particular curvature ratio define a ground-contacting area which is remarkably narrower than a corresponding tire for motor-vehicle. This means that the problems of wet ground grip and aquaplaning, directly related to the size of the ground-contacting area of the tread on the ground, affect the tires in a different way and are therefore solved in a different way according to the type of tire concerned.
In particular, the Applicant has sensed that a tread pattern for motor-cycles, optimized to solve the specific problems of this type of tire (for instance, the excessive wear of the equatorial zone of the tread), but not other much less felt problems (for instance, water draining off), can be adopted as a reference to realize a tread pattern for motor-vehicles.
In fact, the Applicant has understood that the realization of a tire with a tread pattern having no longitudinal grooves might be very advantageous, and in contrast to the constant teaching of the prior art, it has surprisingly found that, by suitably modifying a tread pattern which was considered to be fit for use only in motor-cycles, it was possible to satisfy the different more specific requirements of a tire for motor-vehicles, such as for instance the grip on wet ground and aquaplaning.
The technical problem underlying the present invention is that of providing a tire having structural and functional features adapted not only to ensure an adequate performance of the tire, but also to maintain substantially constantxe2x80x94independently of the wear conditions of the treadxe2x80x94the performances of the tire in general and, in particular, grip on dry and wet ground, tractivity, side stability and noisiness.
According to a first aspect of the invention, this problem is solved by a tire of the aforementioned type, which is characterized in that said transversal grooves are circumferentially distributed along the tread in groups alternately extending from said shoulder zones of the tread, said groups of grooves defining in the equatorial zone of the tread a plurality of substantially continuous portions of the tread ending at the equatorial portion of a same transversal groove of the axially opposed group of grooves.
According to a second aspect of the invention, this problem is solved by a tread for vehicle tires, in particular a premolded tread for covering worn tires, comprising a plurality of transversal grooves including a first shoulder portion and a second equatorial portion formed in an equatorial zone of the tread extending on either side of the equatorial plane of the tire and along at least one of two adjoining shoulder zones defined in axially opposed parts with respect to said equatorial zone, which tread is characterized in that said transversal grooves are circumferentially distributed in groups alternately extending from said shoulder zone of the tread, said groups of grooves defining in the equatorial zone of the tread a plurality of substantially continuous portions of the tread terminating at the equatorial portion of a same transversal groove of the axially opposed group of grooves.
In the following description and in the appended claims, the term: xe2x80x9csubstantially continuous tread portionxe2x80x9d is intended to indicate a portion of the tread which is not interrupted by grooves even though it is crossed by slits however oriented.
According to the invention, the Applicant has found in particular that the aforesaid groups of substantially continuous tread portions alternately extending from opposite shoulder zones towards the equatorial plane of the tire, form a sort of xe2x80x9cgridxe2x80x9d or xe2x80x9cmatrixxe2x80x9d of elastomeric material portions fitted in with one another and substantially devoid of longitudinal hinge elements.
Besides, the substantially continuous tread portions constitute as many groups of xe2x80x9cstrutsxe2x80x9d adapted to discharge, along their own axis and towards the back of the substantially continuous axially opposed portions, the stresses imparted thereto during the tire rolling.
The structural stiffness resulting from the mutual fitting of the substantially continuous axially opposed tread portions, allows these portions to absorb without bending nor too much deforming themselves, all the thermal-mechanical stresses imparted thereto during the tire rolling.
Thanks to such mobility reduction of the various tread portions, it has been noticed a drastic reduction in the thermal-mechanical degradation phenomena of the elastomeric matrix of the tread also in the presence of extreme stresses.
Preferably, the equatorial zone of the tread concerned by the equatorial portion of the transversal grooves extends on either side of the equatorial plane of the tire for a portion having a width comprised between 20% and 60% of the axial development of the tread, said development being understood as a peripheral development of the tire tread mounted on the nominal rim, inflated up to the running pressure and not loaded. This development essentially coincides with the base development of the mold sector used for manufacturing the tire.
Still more preferably, such equatorial zone extends on either side of the equatorial plane of the tire for a portion having a width comprised between 25% and 55% of the axial development of the tread.
Alternatively, said equatorial zone extends on either side of the equatorial plane of the tire for a portion having a width comprised between the planes passing through the points of the equatorial portions of the transversal grooves that are axially farthest with respect to the respective shoulder zone from which they extend.
In the following description and in the appended claims, the term: xe2x80x9csubstantially rectilinear elementxe2x80x9d is intended to indicate either a geometrically rectilinear portion (a segment of a straight line), for instance of a groove or of elastomeric material, or a portion however curvilinear such that the development of the portion does not exceed more than 10% of the straight distance between the start and the end points of said portion.
Preferably, to evaluate if an element extending within the equatorial zone is substantially rectilinear according to such definition, the segment that links the intersection point of the element with one of said circumferential planes defining the equatorial zone and the axially farthest point of the element with respect to said plane, is identified. The length of such segment is then compared with the straight distance that links said points.
In the presence of elements not substantially rectilinear, which do not allow to measure angular values, such measurement is carried out with reference to the straight segment which links the points of the element not substantially rectilinear which are axially farthest within the equatorial zone.
Besides, the transversal grooves preferably impart to the tread pattern a directional characteristic, i.e. they identify a preferred rolling direction of the tire.
In any case, the invention may also be realized with regard to non-directional tread patterns (either symmetric or asymmetric) wherein said preferred rolling direction is not present.
Preferably, the equatorial portion of at least one transversal groove forms an angle (xcex3) with respect to the equatorial plane of the tire comprised between 20xc2x0 and 65xc2x0 (measured both in clockwise and counterclockwise direction, starting from the equatorial plane, according to the inclination of the transversal grooves) and, still more preferably, between 30xc2x0 and 50xc2x0.
Still more preferably, the equatorial portion of at least one transversal groove forms an angle (xcex1) of a width comprised between 40xc2x0 and 130xc2x0 and, still more preferably, between 60xc2x0 and 100xc2x0, with respect to the equatorial portion of a groove of the axially opposed group and extending from the opposite shoulder zone of the tire.
Preferably, such angular values are constant within the equatorial zone, to the advantage of the symmetry and isotropy of the matrix of substantially continuous portions of rubber thus defined on the tread.
Tests carried out by the Applicant have surprisingly shown that the aquaplaning behavior of the tire is not significantly affectedxe2x80x94within the aforesaid interval of preferred inclination valuesxe2x80x94by the inclination of the transversal grooves which can effectively drain off the water collected under the ground-contacting area, independently of the value taken up by said angle (xcex3) within said range.
In a preferred embodiment of the invention, the equatorial portion of the transversal grooves of the tires, in particular those to be mounted on the front wheel of a vehicle, forms an angle (xcex3) with respect to the equatorial plane of the tire substantially equal to 45xc2x0, so as to be substantially perpendicular (xcex1=about 90xc2x0) to the equatorial portion of the grooves of the axially opposed group extending from the opposite shoulder zone of the tire.
In this way, it has advantageously been found an optimum symmetry and isotropy of the matrix of substantially continuous portions of the rubber composition thus defined on the tread, so that the rubber composition reacts in an extremely homogeneous and isotropic way, in particular on the steering tire that is usually most stressed during rolling, whatever the direction of the stress the tire has to withstand and without negatively affecting wet road performances.
With such symmetric configuration of the tread pattern, it has in particular been noticed a so-called xe2x80x9cneutralxe2x80x9d behavior of the tire to the various stresses with the absence of any preferred releasing direction of the latter.
Further important and advantageous effects obtainable thanks to the particular arrangement of the transversal grooves with the formation of a substantially isotropic matrix of portion of elastomeric material fitted into one another include:
a) the achievement of a better wear regularity, that can also be associated to some extent to the stiffness increase of the substantially continuous tread portions, since the tread reacts in the same way whatever the direction of the stress the tire has to withstand; and
b) the possibility of using softer rubber compositions while maintaining the same abradibility values, to the advantage of rolling noiselessness and comfort.
In a further embodiment of the invention, the equatorial portion of at least one transversal groove of the tires, in particular those to be mounted on the rear wheel of a vehicle, preferably forms an angle (xcex3) substantially equal to 30xc2x0 (xcex1=60xc2x0) with respect to the equatorial plane of the tire.
In this way, it has advantageously been achieved the highest performances of the rear tire tread along the direction of greatest stress (traction/braking) during the vehicle""s rolling.
Preferably, at least one of the transversal grooves comprises an equatorial portion extending on either side of the equatorial plane of the tire. In this way, it has been found an optimum aquaplaning behavior of the tire, even in the absence of longitudinal grooves usually adopted in car tires according to the prior art.
Preferably, at least one of the transversal grooves comprises an equatorial portion extending in a substantially rectilinear way within an equatorial half-zone of the tread, meaning by this term the portion comprised between the equatorial axis and the starting point of one of the shoulder portions of the tread itself.
According to an alternative embodiment of the invention, wherein the equatorial zone of the tread has a limited width (for instance comprised between 35% and 45% of its axial development defined above), at least one of the transversal grooves comprises an equatorial portion extending in a substantially rectilinear way at least in part within one of the shoulder zones of the tire tread.
Preferably, in each group of grooves, the grooves have a length that decreases along the rolling direction of the tire, and comprise equatorial portions at least in part substantially parallel to one another, to the advantage of the symmetry and isotropy of the equatorial zone of the tread.
Preferably, the equatorial portions of the transversal grooves terminate at a prefixed distance from the equatorial portion of a same transversal groove, still more preferably of the transversal groove having a prevailing length of the axially opposed group of grooves. Preferably, such distance is comprised between 0 mm and 50% of the mean pitch of the tread pattern and, still more preferably, it is at least equal to 4 mm.
In the following description and the in appended claims, the term: xe2x80x9cmean pitchxe2x80x9d of the tread pattern is intended to indicate the arithmetic mean of the distances between consecutive transversal grooves, measured along the circumferential development of the tread.
Preferably, the transversal grooves comprise two substantially rectilinear portions extending along the equatorial zone and, respectively, the shoulder zone of the tire, connected by means of a substantially curvilinear intermediate length having a curvature radius comprised between 30 and 60 mm.
Such curvature radius varies according to the chord of the tire and may be easily selected in such range of values by those skilled in the art.
Preferably, the shoulder portion of the transversal grooves forms with the equatorial plane of the tire an angle (xcex2) of a width comprised between 85xc2x0 and 95xc2x0 and, still more preferably, equal to about 90xc2x0.
In a preferred embodiment, the equatorial portions and at least a part of the shoulder portion of the transversal grooves have a substantially constant width along the tread portion that substantially corresponds to the ground-contacting area of the tire along straight stretches.
In this way, the drain off of water present under the ground-contacting area of the tire is advantageously increased, with a reduction of aquaplaning phenomena
Preferably, the width of the equatorial portions and of at least a part of the shoulder portion of the transversal grooves is comprised between 5 and 10 mm.
The optimum width value may be easily selected within such range by those skilled in the art, according to the specific pitch of the grooves comprised in a given group, meaning by this term the distance, measured along the circumferential development of the tread, between two consecutive transversal grooves.
In a preferred embodiment of the invention, the width of the equatorial portions and of at least a part of the shoulder portion of the transversal grooves is comprised between 6 and 8 mm.
In this way, an optimum balancing has been found between the noisiness reduction and tire performance on wet ground.
In a further embodiment of the invention, the shoulder portion of the transversal grooves comprises at one end a constriction or end portion having a reduced width with respect to the remaining part of the same.
Preferably, such end portion has a width comprised between 40% and 60%, and preferably about 50%, of the maximum width of the transversal grooves, and allows to achieve the following advantages:
1) an improvement of road holding in curves and extreme driving conditions, such as for instance those associated to the sports use of the tires on high-performance vehicles, thanks to the greater stiffness of the tread portions included in the shoulder zones and to the greater solid area under the ground-contacting area in curves;
2) a drastic limitation of the so-called xe2x80x9csaw-toothxe2x80x9d irregular wear phenomena of said portions included in the shoulder zones, thanks to the reduction in wear non-uniformity between the leading and trailing edges of the grooves having a reduced width.
Preferably, such portion is located outside of the ground contacting-area of the tire along straight stretches, so as not to hinder a regular water draining from the equatorial zone of the tread towards the outside, while it falls within the ground-contacting area in curves or drift rolling, allowing to achieve the aforementioned improvement.
In a preferred embodiment of the invention, there is an odd number of grooves in each of the above groups, to improve symmetry and isotropy of the tread pattern.
Preferably, each of the groups of grooves formed in the tread comprises from 3 to 7 and, still more preferably, from 3 to 5 transversal grooves.
In an embodiment of the invention, the front and rear tires have the same chord: in this case, the tread of the front tire is provided with groups preferably comprising three transversal grooves, while the tread of the rear tire is provided with groups preferably comprising five transversal grooves.
In an alternative embodiment, the rear tire has a chord greater than the chord of the front tire: in this case, the tread of both tires is provided with groups preferably comprising five transversal grooves.
Preferably, the transversal grooves have a depth comprised between 5 and 9 mm and, still more preferably, between 6.5 and 8.5 mm, independently of their number and of the tire size.
In a preferred embodiment, the transversal grooves of each of said groups are longitudinally staggered with respect to the grooves of the axially opposed group by a length equal to 50% of the mean pitch of the tread pattern.
In this way, it has been found an advantageous reduction of the tire noisiness, which has also been optimized thanks to the tread pattern having a special conformation by groups of grooves.
In fact, in the tires of the invention it is possible to reduce the overall noisiness of the tread pattern operating in two ways, namely by regulating either the pitch value between individual adjoining grooves or the pitch value between groups of consecutive grooves, meaning by this term the distance, measured along the circumferential development of the tread, between corresponding transversal grooves (for instance, those of greatest length) of consecutive groups of grooves.
Therefore, the pitch between consecutive groups of grooves corresponds to xe2x80x9cnxe2x80x9d times the pitch between individual grooves, wherein xe2x80x9cnxe2x80x9d is the number of grooves comprised in each group.
In the tires of the invention it is therefore possible to reduce not only the noisiness corresponding to the wavelengths between the 50th and the 70th harmonic (meaning by 1st harmonic that of the wheel turn) which are a function of the pitch between the individual grooves, but also the noisinessxe2x80x94the optimization of which proves to be very difficultxe2x80x94corresponding for instance to the wavelengths between the 10th and the 25th harmonic, which, instead, is a function of the pitch between the groups of grooves.
Advantageously, by changing the number of the transversal grooves comprised in each group, it is also possible to have a more or less high number of groups to be circumferentially distributed along the tread pattern, obtaining in this way a high design flexibility of the pitch sequence, to achieve the lowest overall noisiness.
So, for instance, it is possible to optimizexe2x80x94if desiredxe2x80x94the noisiness corresponding to the wavelengths between the 10th and the 14th harmonic by using groups comprising five transversal grooves, or to optimize the noisiness corresponding to the wavelengths between the 16th and the 23rd harmonic by using groups comprising three transversal grooves. Besides, in the latter case, the greater number of groups of grooves allows a greater design freedom to optimize the acoustic result.
In a further embodiment of the invention, the tire tread may possibly comprise a central depression preferably formed astride the equatorial plane of the tire and, still more preferably, circumferentially extending without interruptions.
This feature is particularly advantageous since it allows to keep substantially unchanged the reaction of the tread pattern to the different stress conditions, so that the groups of xe2x80x9cstrutsxe2x80x9d will continue to release the stresses along their own axis and towards the back of the essentially continuous axially opposite portions.
In fact, by considering simply the aspect of an improvement of wet road behavior, such central depression might be adequately replaced by a longitudinal groove. However, such a groove would interrupt the isotropy of the pattern, concentrating against the wall of said groove the release of the stresses and reducing in this way the advantages of the substantially isotropic matrix synergistically defined by the groups of struts.
Preferably, said central depression has a flared substantially V-shaped transversal section and has a variable depth, which increases towards the equatorial plane of the tire wherein it reaches its maximum value, which is comprised between 40% and 60% of the transversal grooves depth. Besides, the central depression has a width preferably comprised between 15 and 25 mm.
In this way, it has been noticed an advantageous improvement in wet road behavior, with a reduction of aquaplaning phenomena.
Preferably, the tire tread according to the invention further comprises a couple of longitudinal slots or xe2x80x9cdisconnection groovesxe2x80x9d, extending from opposite parts of the equatorial plane of the tire on said shoulder zones.
Preferably, furthermore, the longitudinal slots extend substantially throughout the whole circumferential development of the tread.
Advantageously, said slots contribute to impart to the tread an adequate flexibility at the shoulder zones of the tire, recovering their curvature effect and increasing the ground-contacting area.
In a preferred embodiment, said longitudinal slots have a depth equal to or lower than 2 mm.
Preferably, the tire according to the invention further comprises a couple of longitudinal continuous slits circumferentially extending in the shoulder zone and on eother side of the equatorial plane of the tire.
Advantageously, said slits contribute to reduce the noisiness of the tire, and to increase the ground-contacting area of the tire, recovering their curvature effect and increasing road holding on wet ground.
In a preferred embodiment, the longitudinal slits comprise a plurality of adjoining portions, circumferentially extending without interruption, each having a variable depth comprised between 1 and 9 mm and a width not greater than 2 mm.
Preferably, the tire according to the invention further comprises a plurality of longitudinal slits of a prefixed length, circumferentially extending in their own equatorial zone and on either side of the equatorial plane of the tire.
Additional slits, also having a prefixed length, span in the equatorial zone at the equatorial plane of the tire along which they are distributed according to a variable pitch.
Advantageously, said slits contribute, on the one hand, to reduce the noisiness of the tire and, on the other hand, to improve road holding on wet ground and the plastic and acoustic comfort during rolling, besides ensuring a more uniform wear regularity of the rubber.
Such slits may be perpendicular, parallel or anyhow inclined with respect to the transversal grooves and preferably have a width comprised between 1 and 2 mm.
Preferably, furthermore, the slits have a depth variable in a direction opposite to the rolling direction of the tire and comprised between 1 and 9 mm.
Advantageously, the variable depth of said slits contributes to obtain a more regular and uniform wear of the tread.
Preferably, the tire of the invention further comprises a plurality of transversal notches formed in said shoulder zones of the tread and located between adjoining transversal grooves.
Advantageously, said notches contribute both to reduce the noisiness of the tire by distributing the noise caused during rolling throughout the whole range of the audible frequencies, and to increase the curve traction characteristics of the tire.
In a preferred embodiment, said notches have a depth comprised between 3 and 4.5 nm and a width comprised between 2 and 3.5 mm; preferably, furthermore, said notches extend from the axially outer edge of the tread pattern up to the corresponding continuous longitudinally slit.
According to a third aspect, the invention also relates to a set of tires comprising a first couple of tires adapted to be mounted on the front wheels of a vehicle and a second couple of tires adapted to be mounted on the rear wheels of a vehicle, said first and second couple of tires comprising a first and respectively a second tread on each of which are defined:
i) an equatorial zone extending on either side of the equatorial plane of the tire, and
ii) two shoulder zones in axially opposite positions with respect to said equatorial zone;
said first and second treads being provided with a plurality of transversal grooves including a first shoulder portion and a second equatorial portion, which set is characterized in that:
1) in the front tires said transversal grooves are circumferentially distributed along the first tread in groups alternately extending from opposite shoulder zones of the tread, each of said groups including from three to five grooves,
2) in the rear tires said transversal grooves are circumferentially distributed along the second tread in groups alternately extending from opposite shoulder zones of the tread, each of said groups including from five to seven grooves,
said groups of grooves defining in the equatorial zone of said first and second treads a corresponding plurality of substantially continuous portions of the tread terminating at the equatorial portion of a same transversal groove of the axially opposite group of grooves.
In an embodiment of the invention, the front and rear tires have the same chord: in this case, the tread of the front tire is provided with groups preferably comprising three transversal grooves, while the tread of the rear tire is provided with groups preferably comprising five transversal grooves.
In an alternative embodiment, the rear tire has a chord greater than the chord of the front tire: in this case, the tread of both tires is provided with groups preferably comprising five transversal grooves.
In a preferred embodiment of the invention and as described above, the transversal grooves of the tires may form angles (xcex1), with respect to the equatorial portion of the grooves of the axially opposed group, which could be different in the front and the rear tires: in this case, such an angle is substantially equal to 90xc2x0 in the tires of the front couple (steering tires) and substantially equal to 60xc2x0 in the tires of the rear couple.
Preferably, the tread of the front tire further comprises the central depression formed astride the equatorial plane of the above described tire, so as to improve wet ground behavior and in particular to reduce aquaplaning phenomena.
In a further embodiment of the invention, in one or both the front and rear tires, the shoulder portion of the transversal grooves comprises at one end a constriction or end portion having a reduced width with respect to the remaining part of the same.
Preferably, said end portion has a width comprised between 40% and 60%, preferably about 50%, of the width of the transversal grooves and allows to achieve the advantages described hereinabove.
In testing the tires of the invention on a sports-car, the Applicant has found that, contrary to all expectations, the best results on dry ground were achieved by mounting on the front steering axis of the car a couple of tires having a tread pattern showing directional characteristics less marked with respect to the tread pattern of the tires mounted on the rear tractive axis. Still more surprisingly, it has been found that, during the tests on wet ground, the inversion of the tread patterns between the front and the rear tires did not substantially modify the performances of the car.
To evaluate the directionality characteristics of a tire it is sufficient to mount the tire with a rolling direction opposite to the preferred one and to record the performances degradation. Generally, higher directional characteristics correspond to tread patterns having transversal grooves which form angles having smaller values with respect to the equatorial plane.
According to a fourth aspect, the invention relates to a method of optimizing the road behavior of a vehicle equipped with a set of tires, comprising the following steps:
a) mounting on the front steering axis of the vehicle a couple of tires comprising a first tread allowing a substantially neutral behavior to lateral and longitudinal stresses and combinations thereof;
b) mounting on the rear tractive axis of the vehicle a couple of tires comprising a second tread having a preferred releasing direction of longitudinal stresses with respect to lateral stresses.
The Applicant believes that this performance increase is due to the fact that the tires mounted on the front axis of the vehicle undergo to greater lateral stresses with respect to the rear tires upon entering a curve, since they must transmit the vehicle change of direction, while the rear tractive tires undergo to greater longitudinal stresses to discharge down to the ground the torque transmitted by the engine.
Therefore, it is convenient to adopt on the front tire a neutral pattern that does not hamper the behavior to side thrusts to the advantage of longitudinal thrusts. In its turn, the rear tire may be advantageously specialized to improve its response to the stresses it is prevailingly submitted to.
Advantageously, the front tires may be provided with a depression improving the performances on wet ground, without modifying the grid or matrix of grooves and struts, therefore maintaining the neutral behavior of the tire associated to the maximum pattern isotropy.